In these days, the near field communication technology of signal transmission and reception by electromagnetic induction has been established and its use has extended in the form of tickets for public means of transit or electronic money. The function of near field communication tends to be loaded on mobile phones as well and its use in time to come is felt to be promising. The near field communication technology is not limited to proximity communication by electromagnetic induction, such that, in the field of logistics, an IC tag that enables read/write at a distance of several meters has been commercialized. The near field communication technology not only enables near field communication, but also provides for power transmission at the same time. Consequently, the technology may be implemented on an IC card which does not own its own power supply, such as a battery.
In a system that implements the above mentioned near field communication, near field communication and power transmission are effected between a reader/writer and a wireless data carrier. To this end, a capacitor for resonance is connected to a loop antenna, and the resonance frequency, as determined by a constant LC of the loop antenna and the capacitor, is tuned to a preset system frequency. By so doing, stabilized communication may be established between the reader/writer and the wireless data carrier at a maximized communication distance.
However, the constant LC of the loop antenna and the capacitor for resonance has a number of factors of variations and may not necessarily be set at a scheduled value. For example, in the wireless data carrier, the loop antenna is formed by a copper foil pattern to reduce the cost, such that the value of L is varied due to, for example, deviations in pattern widths. Similarly, the capacitor for resonance is formed with the use of a copper foil of an antenna board as an electrode and with the use of the resin of the board as a dielectric material, again to reduce the cost. Hence, the capacitance is changed with the width, length or the pitch of the copper pattern. On the other hand, a protective film is finally laminated on each of upper and lower sides of the antenna board for use of the antenna board as an IC card. However, the capacitance is varied under the influence of the protective film. Thus, to take the frequency shift following the lamination of the protective film into account, the copper foil pattern is partially removed by way of prospective adjustment with a view to adjusting the electrode area as well as the capacitance value of the capacitor for resonance.
The above mentioned factors of variations may give rise to shifting of the resonance frequency to destabilize communication or to reduce the communication distance. To cope with such problem, Patent Document 1 shows, in connection with an antenna module, a method of adjusting the resonance frequency by adjustment of the capacitance of the variable capacitor to provide for stability in communication. The antenna module includes an antenna coil that receives magnetic fluxes output from the reader/writer and a resonance circuit that efficiently converts changes in the magnetic fluxes into an electrical voltage.